Miss Shilling’s Orifice Fixed Merlin Engines on Spitfires

Ken Zino of AutoInformed.com on Beatrice Shilling and SU Carburetors During WW2

Beatrice continued racing a 1935 Lagonda at Silverstone and took part in sportscar races at Goodwood during the 1950’s but was always known for her Merlin engine modifications.

My first knuckle-skinning love of SU Carburetters (sic) came along with sundry ground-bound Austin-Healey and MGs I owned and thrashed over New Jersey and New York roads. Little did I know, that late in 1940, years before I was born, an engineer named Beatrice Shilling introduced a modification that allowed the Merlin engine to overcome “fluff” or fuel starvation, thereby allowing war production of automotive parts to defend Old Blighty during the Battle of Britain.

In one of those odd and startling connections, we are in a war now against Covid that will require the collective sacrifice and patriotism that was on display during the British battle and subsequently – in another parallel – our struggles under the Roosevelt Administration that followed a demonstrably incompetent Republican one.

This recent enlightenment about Skinners Union (aka SU) Carburetter (pace critics – two countries separated by a common language) Company known for automotive applications. During the Second World War, SU paused road car carburetter production to help with the war effort.

Burlen* – current owner of Skinners Union (SU) Carburetters established by the Skinner family in 1910 – at year end celebrated Miss Shilling’s Orifice, 80-years after it saved the Merlin engine used in the Hurricane and Spitfire combat airplanes.

Miss Beatrice Shilling, Tilly to her friends, was a remarkable lady who was once described as “a flaming pathfinder of women’s lib.” Born in 1909, she was awarded a gold star in the 1930s for lapping Brooklands on her Norton 500 motorcycle at more than 100 mph. Later Beatrice Shilling would gain an engineering degree at Manchester University and was recruited as a scientific officer by the Royal Aircraft Establishment (RAE), a position she held until her retirement in 1969.

During her RAE career she led investigations into the aquaplaning of aircraft during take-off and landing on wet runways and was awarded an OBE in 1948. Beatrice continued racing a 1935 Lagonda at Silverstone and took part in sportscar races at Goodwood during the 1950’s but was always known for her Merlin engine modifications.

The Spitfire and Hurricane aircraft used early in the war had a Rolls-Royce Merlin III engine fitted and when 100-octane fuel became available it allowed for increased boost, taking power to 1310 hp. At this time all Hurricane and Spitfire engines were fitted with an SU AVT35/135 carburetter.

However, there was  a problem with the SU carburetter when it was subject to negative G. Entering into a steep dive the Merlin engine suffered a ‘fluff’ caused by the fuel level at the jets momentarily being thrown to the top of the float chamber, starving the jet of fuel for ~1.5 seconds. Despite this being a possibly disastrous situation, the carburetters were able to function normally after the floats had regained control.

A solution to this was developed by Beatrice Shilling at the RAE laboratories, offering a preventative modification that would be retrospectively fitted to all serving aircraft. The solution credited to her was a restrictor (or orifice plate) with a calibrated aperture in the center that was attached to the fuel line before the carburetter. It limited fuel flow to a volume only slightly less than the engine demanded at full power and while it did not stop the momentary weak hesitation, it did prevent the 1.5 second rich cut. This modification is known as Miss Shilling’s orifice.

By 1941 the Royal Aircraft Establishment developed an anti-G modification which would be retrofitted to serving aircraft and would also become a standard feature of all new SU carburetters. However, Miss Shilling’s orifice was retained by many aircraft, depending on their role in battle.

“Beatrice Shilling is an inspiration to us all, not only was she a first-class engineer, but she also loved speed and was fast on both two wheels and four. Today Burlen lives by that mantra, combining innovative product engineering and a love of all motor-powered vehicles. Miss Shilling should be celebrated and remembered for the incredible woman she was, and as an engineer who made wartime pilots lives as safe as they could be when in battle,” said Mark Burnett, Burlen Managing Director.

*Burlen Ltd is the world’s sole manufacturer of genuine SU, Amal and Zenith carburetters and owns the J40 Motor Company, dedicated to the preservation of the Austin J40 pedal car.

About Ken Zino

Ken Zino is an auto industry veteran with global experience in print, broadcast and electronic media. He has auto testing, marketing, public relations and communications expertise garnered while working in Asia, Europe and the U.S.
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